1. Field of the Invention
The present invention relates to a device for hot isostatic pressing. A device provided in accordance with one embodiment of the invention includes a pressure vessel having a wall, and a heat-insulating mantle surrounding a furnace chamber. The heat-insulating mantle is located within the pressure vessel walls with a first gap separating the heat-insulating mantle from the pressure vessel walls. The heat-insulating mantle is closed by a top member. The heat-insulating mantle contains a lower mantle opening and an upper mantle opening to the first gap such that a pressure medium enters a cooling loop in the first gap through the upper mantle opening and exits from the cooling loop through the lower mantle opening into the first space located below the furnace chamber.
A first guiding means is adapted to guide the pressure medium, which surrounds a load space in the furnace chamber adapted to receive loads to be pressed. The first guiding means has an upper discharge opening, and a lower entry opening, being arranged to define a second gap between the first guiding means and the heat-insulating mantle. The second gap permits the flow of the pressure medium upwardly through the interior of the load space. Upon the pressure medium reaching the top member of the heat-insulating mantle, the pressure medium is redirected to flow downward through the second gap to a second space located below the load space and above the first space.
The device is also provided with first and second pump means. The first pump means is adapted to pump a cooled pressure medium emanating from the cooling loop upwardly from the first space towards the second pump. The second pump means, which is arranged in the second space, is adapted to pump a mixture of the cooled pressure medium from the first pump means and a warm pressure medium emanating from the second gap upwardly through the load space.
2. Description of the Related Art
Hot isostatic pressing (HIP) is a technique being employed more frequently to press out xe2x80x9cshrinkagesxe2x80x9d in castings through the combination of heat and high gas pressures within a furnace chamber. The HIP technique has been used on turbine blades for aircraft to substantially increase the strength and life of the blades. HIP is also used in manufacturing products made of compact material to minimize the porosity of the final product by pressurizing and heating the product simultaneously.
Typical pressures in hot isostatic pressing are 500 to 5,000 bars, while the temperature usually varies between 500 and 2,200xc2x0 C. The HIP operating temperature is approximately 75% of the fusion temperature of the material being pressurized. An inert gas, such as argon, is usually used as a pressure medium.
In some systems, such as those described in U.S. Pat. No. 5,123,832 of the applicant, a first and second pump, such as ejectors (jet pumps), are driven for cooling loads present in the load space after the heating phase has been terminated. However, the present invention is not restricted to use of such a device for cooling purposes. Also, as will be discussed below, the pressure medium in the present invention may be a fluid or a gas, but for simplifying the description and for illuminating the object of the invention, the pressure medium will be described as a gas hereinafter.
While the ""832 invention functions well, applicant believes it would be beneficial to control the cooling rate of the loads within the HIP device. To obtain preferred results from the hot isostatic pressing process, the cooling should proceed in a controlled manner and at a desired rate, i.e., the number of degrees Kelvin per minute. It is also important that the cooling takes place substantially homogeneously in the entire load space, such that each load acquires the same properties. The movement of the gases up through the load space is opposite to the direction of natural convection, since the gases entering the load space have a lower temperature than the loads and are gradually heated as the gases transition up through the load space. The driving mechanism for the gas to move through the load space and then circulate downward through the second gap is the impulse provided by the second pump means. The gas flow would completely stop if the first pump means, which drives the second pump means, were switched off for a short period. Consequently, defects in the properties of the loads would result. The flow of the first pump means and the impulse of the second pump means have to be regulated. Although it is possible to regulate the first and second pump means in steps, this regulation is not sufficiently precise and very inert, since it takes a considerable amount of time for the gases to make the transition through the load space after a change in the power of the first pump means. Furthermore, the impulse generated by the second pump means must be substantial enough to overcome the natural convection of the gases, so that the cooling rate may be regulated within a restricted interval.
The object of the present invention is to provide a device, which more accurately regulates the temperature of the pressure medium pumped by the second pump means.
According to the present invention, a third pump means can be arranged in the second space to pump a warm pressure medium, emanating from the second gap, to the second pump means. A control unit can also be used to successively switch the first and third pump means on and off.
Utilizing the third pump means and the control unit to switch the first and third pump means on and off allows for more accurate temperature and pressure regulation of the pressure medium pumped by the second pump means through the load space. Thus, when the first pump means is switched off, and the third pump means is switched on, the warm pressure medium emanating from the second gap will be pumped through the second pump means upwards through the load space. In the case of cooling the loads, the cooling effect will be less efficient if only the first pump means is driven.
However, a good stirring and transporting of the pressure medium up through the load space is ensured by the ability of the second pump means to give the pressure medium a sufficient impulse through the third pump means. Thus, it is possible to accurately regulate the cooling effect by alternatingly switching the first and third pump means on and off through the control unit without disturbing the stirring of the pressure medium in the load space. Disturbing the stirring of the pressure medium within the load space may cause non-uniformities of the products pressed.
The cooling effect may be controlled by the length of the periods of time during which the respective pump means are switched on. However, the field of use of the present invention is not in any way restricted to cooling. It would also be possible to keep the first pump means switched off and the third pump means switched on during the entire or parts of the heating phase of the hot isostatic pressing. In this manner, a good stirring of the pressure medium and by that a good uniformity of the influence thereof upon the loads results in uniform properties of the loads independent of the location of the loads within the load space.
According to a preferred embodiment of the invention, the device is adapted to cool the loads after a terminated heating phase. The second pump means is adapted to pump a pressure medium from the second space upward through the load space. The pressure medium, upon leaving the second pump means, has a lower temperature than the temperature of the loads. Therefore, the cooling cycle occurs from heat transfer from the loads to the pressure medium as it moves up through the load space. The control unit is adapted to alternatingly switch the first and third pump means on and off, so that only one of them at a time is pumping the pressure medium to the second pump means. The temperature may therefore be accurately regulated without disturbing the stirring through the second pump means of the gas in the load space. The third pump means drives a warm pressure medium to the second pump means without giving any substantial cooling effect and whereas the first pump means drives a cooled pressure medium to the second pump means, therefore generating a greater cooling effect on the loads.
According to another embodiment of the invention, the control unit is adapted to keep the third pump means switched on and the first pump means switched off during the heating phase of the hot isostatic pressing for stirring the warm pressure medium. It may be possible to utilize the second pump means to ensure a good stirring of the pressure medium in the load space during the heating phase, such that the second pump means by this will have a broadened field of use and may contribute to improving the uniformity of the temperature upon the loads during the heating phase of the hot isostatic pressing.
According to another embodiment of invention, the pressure medium is an inert gas and the second pump means is a jet pump. The second pump means can be adapted to pump warm gases emanating from the second gap through jets from the first pump means and the third pump means. It is by this possible to draw in considerably larger flows of warm gas emanating from the second gap into the second pump means through comparatively small flows from the first and third pump means and bring these flows into circulation passing the loads, and downwardly in the second gap, so that a good stirring is obtained and the temperature difference between the gases passing the loads and the loads will not be too large.
According to another embodiment of the invention, the first pump means and the second pump means are jet pumps. The jet pumps are arranged to be driven through jets of gas, which emanate from a compressor arranged outside the pressure vessel. The gas from the compressor has a lower temperature than the warm gas from the second gap. The first and third pump means may advantageously be driven by such cold gases. Because the cooler gases from the first and third pump means have a temperature too low to allow them to directly contact the loads, the cooler gases are driven through the second pump means prior to entering the load space, The second pump means combines the cooler gases with warmer gases, the cooler gases constituting only a fraction of the total mass flow, such that the temperature of the gases entering the load space is acceptable. A thorough mixing of the flows emanating from the first and third pump means with the warm gas emanating from the second gap also occurs within the second pump means. In the instance when only the third pump means is switched on during the heating phase, the majority of the flowing gases delivered to the load space will be formed by the warm gases. In turn, these flowing warm gases will generate an even larger flow of warm gases from the second gap out of the second pump means and into the load space. Therefore, the cold gas introduced into the third pump means will not significantly influence the temperature of the gases entering the load space from the second pump means.
According to another embodiment of the invention, the first and third pump means are connected to the same compressor, and the control unit is arranged with a control valve at the outlet of the compressor. The control valve controls the flow of cold gas from the compressor to the first or third pump means. This simple and reliable arrangement indicates the ability of the device to readily regulate temperature. The desired practice is to have only the first pump means or the third pump means switched on at any given time; however, it is conceivable to have both pumps switched on at the same time. Furthermore, the use of only one compressor for both the first and third pumps simplifies the device and the control operation.
According to another embodiment of the invention, the device comprises a bottom plate arranged to separate the first and the second space. The bottom plate keeps the flow of the warm pressure medium from the second gap and the flow of the cooled pressure medium from the first gap from interacting with each other. A well-controlled regulation of the pressure medium is obtained through the second pump means by controlling the amounts of warm and cold pressure medium emanating from the third and the first pump means, respectively.
According to another embodiment of the invention, the third pump means is arranged in the bottom plate and has an outlet directed upward. Furthermore, the device comprises a second channel means extending from the second space into the bottom plate for the supply of the warm pressure medium emanating from the second gap to the third pump means. The bottom plate may be used to advantageously conduct the warm pressure medium into the third pump means for pumping it through the third pump means to the second pump means.